The study is concerned with the course of infection in two inbred mouse strains of two T. cruzi clones and a strain isolated from an acute human case of American Trypanosomiasis. Through the use of cloned stocks the existence of intrastrain subpopulations differing in their pathogenicity for an inbred strain was intermediate between that of the two clones. The inbred mouse-T cruzi clone is a highly stable and reproducible experimental system.